Accelerando
/əkˌseləˈrændəʊ/
Definition 1 of 2
Pro Tip 1/3Music Direction
Used by composers to direct musicians to gradually increase tempo. 
There's an 'accelerando' after the intro, so keep pace with the drums.
Pro Tip 2/3Emotional Intensity
Often signifies a build-up of emotion or tension in the music. 
Feel the 'accelerando' as the music sweeps you up in the excitement.
Pro Tip 3/3Not Sudden
The speed increases gradually, not instantly or abruptly. 
From the 'accelerando' cue, slowly pick up the pace.